The present invention relates to an apparatus for the determination of the nature of the earth formation by neutron well logging. More particularly, the invention is concerned with improvements in neutron well logging with the use of a pair of substantially identical scintillation or Neutron-Gamma (N-G) detectors having similar function and operation, and each detecting spectral capture gamma radiation representing hydrogen and chlorine for determining the presence of hydrocarbon or salt water in the earth formations traversed by a borehole.
Neutron well logging techniques and methods have been used to analyze earth formations along the traverse of a borehole for over 30 years. Two such techniques and methods used in the prior art for determining the presence of hydrogen and chlorine in a porous earth formation are embodied and accomplished as set forth in U.S. Pat. No. 3,219,820 and U.S. Pat. No. 3,772,513. Both patents are primarily concerned with acquiring a radioactive measurement indicative of hydrogen, formation reference signal (H signal) or (FR) signal which is substantially insensitive to chlorine in the formation; and hydrogen plus chlorine, formation reference signal plus chlorine signal (H+Cl signal) or (FR+Cl) signal which is sensitive to chlorine in the logged formation as well as being indicative of hydrogen. By comparing these measurements relative to the respective embodied patents, both techniques and methods could be used to determine the presence of hydrocarbon or salt water in a porous formation.
It is an object of the present invention to provide improvements in the methods and apparatus disclosed in the two aforementioned patents, particularly the spectral capture gamma method in that of U.S. Pat. No. 3,772,513. It is also the object of the present invention to provide an improved well logging apparatus directed towards spectral determination of hydrogen and chlorine of earth formations for the subsequent determination of the presence of hydrocarbon or salt water in a porous formation traversed by a borehole by utilizing two substantially identical N-G detectors. The detectors are insensitive to adverse effects of certain other interfering elements, which may be present in the earth formation and borehole, and can be used for lithology differentiation of the earth formations.
The present invention involves improvements in neutron well logging methods and apparatus, which may be embodied in a logging system comprising an instrument, which includes a neutron source for irradiating earth formations as it is passed along the borehole. Spaced at predetermined distances from the source in the instrument along the longitudinal axis of the borehole, there are two substantially identical N-G detectors, each detecting an aggregate capture gamma radiation spectrum from the adjacent formation. The arrangement of the two N-G detectors with respect to distance from the source designates the closest N-G detector as the Near detector and the other N-G detector as the Far detector.
The aggregate capture gamma radiation spectrum from each N-G detector is simultaneously proportioned within two separate predetermined energy ranges, one of which, referred to as Hydrogen or formation reference signal, is indicative of hydrogen and insensitive to chlorine in the formation, and the other of which, referred to as Chlorine or formation reference plus chlorine signal, is indicative of both hydrogen and chlorine in the adjacent formation. The two N-G detector arrangement with respect to distance from the source designates the proportioned energy ranges for the aggregate capture gamma radiation spectrum from the closest N-G detector as Hydrogen Near and Chlorine Near; and the proportioned energy ranges of the other aggregate capture gamma radiation spectrum from the other N-G detector as Hydrogen Far and Chlorine Far.
Surrounding each of the N-G detectors, there is provided a shield of selected material having a high capture cross-section for neutrons and characterized by having a significant thermal neutron induced (capture) gamma radiation energy response within the predetermined Hydrogen energy range of the aggregate spectra of both N-G detectors and having an insignificant gamma radiation energy response within the predetermined Chlorine energy range of the aggregate spectra of both N-G detectors. The selected material of the shield and the functionality of the shield are explained in U.S. Pat. No. 3,772,513 and other preferred embodiments.
Accordingly, since the apparatus of this present invention uses a pair of substantially identical N-G detectors having substantially identical function and operation, each surrounded by substantially identical shields, then the neutron source to detector spacing introduces, in the borehole and earth formations, differing neutron flux in the vicinity of the differently spaced N-G detectors adjacent to the earth formations. It is the object of the present invention to utilize Hydrogen Near, Chlorine Near, Hydrogen Far, and Chlorine Far with respect to the aforementioned differences to give an indication of hydrocarbon or salt water in earth formations traversed by a borehole. It is also the object of this invention to utilize Hydrogen Near, Chlorine Near, Hydrogen Far, and Chlorine Far with respect to the aforementioned differences to give an indication of lithology differentiation in earth formations traversed by a borehole.
It is also the object of the present invention to record Hydrogen Near, Chlorine Near, Hydrogen Far, and Chlorine Far data, in the apparatus, to memory as a function of radiation intensity with respect to time that is later retrieved at the surface along with the apparatus.